Fortification Friday: Dead space is a dangerous angle for the defender

Last week we looked at faces, flanks, and curtains of a fortification.  And with that discussion, we began to look at the geometry outside the fort which depicted the areas covered by the defenders.  In particular, we saw how Mahan demonstrated the angle of the salient within the plan:


For any given salient angle, there is a line to depict the orientation of the salient.  This line ran through the point where the faces intersected, and perfectly split the angle… or as Mahan described:

The line bisecting a salient angle is denominated the capital.

We see two capitals in Mahan’s textbook diagram:


The point at which the capital crossed the line of the fortification was an important feature.  That intersection determined the forward-most defensive point on the salient.  Taken in conjunction with the complementing salients (to the left and right), this set a forward-facing imaginary line (outside the physical lines of fortifications, that is).  Mahan gave this a name and definition:

… the distance from a salient to its opposite flank is a line of defense.

Here is that line on the diagram:


Note that the line of defense is in this case parallel to the curtain.  That was not always the case, as situations might demand asymmetrical salients.  But going back to the nature of the layout with salients, angle of defense, and other components in order, the line of defense was in front of the curtain.  In short, the line of defense depicted the line on which the defender hoped to resist the advance of the attacker.  Consider how the two angles of defense served to provide a cross fire across that line:


Yes, a practical application of geometry there.

While that is fine to show where the defenders would shoot at the “bad guys.”  There had to be a converse to that aspect of the defense – the places where the defenders could not engage the “bad guys.”  In other words the natural defects of a defense:

The form of the parapet, and the direction in which a soldier naturally aims in firing over one, are the causes of two of the most important defects of intrenchments.  Owing to the form of the parapet and its height, the fire can take effect only at some distance beyond it, so that when the enemy has approached very near the parapet, particularly when he is in the ditch, the fire will pass over his head, unless the flanks are so arranged that their fire will sweep every point of the ditch; an arrangement of which particular angular systems are alone susceptible.  This space, where the enemy can find a shelter, is, generally, in the ditches at the re-entering angles.   It is denominated a dead space, or dead angle.

We discussed one aspect of this in relation to the planing of the profile:


But now we are matching that in with the layout on the horizontal plane.  Let me depict that on Mahan’s diagram:


Consider the play of the two causes cited by Mahan here.  Because the defender on a given face cannot aim below the crest of the parapet, he cannot engage an enemy in the ditch to his front.  And this space just happens to be outside of the area covered by the opposite face’s angle of defense. This meant the ditch directly in front of a face along with the ditch along a curtain were within the defined dead space.  This is depicted with the red shaded line, in the figure above.  To mitigate the dead space allowed by the works, the engineer had to carefully arrange the salients with respect to the curtain, thus minimizing the dead space encountered at the re-entering angles.

But that was not the only dead space to consider.  Mahan noted another issue related to the orientation of the faces:

In delivering his fire a soldier usually aims directly to the front, so that the line of fire and the parapet make nearly a right angle with each other.  In consequence of this the salients receive no protection from themselves, and there is angular space in front of each of them (which is equal to the supplement of the salient angle) that is defended only by the fire of flanks.  This space is denominated a sector without fire.

Going again to the diagram, here are sectors without fire:


Note how the capital line also bisected the sector without fire.  It’s a geometry thing again, and a good reference line.  This flaw was not quite so bad as dead space, since the complementing salients had the area under the angle of defense… though at longer range.

What should be apparent in this discussion is how important the measure of those angles were for the arrangement of defense.  If any of these angles were too extreme (too wide, or too narrow) that would introduce demands on the adjacent sectors of the defense.  If those demands were not provided for – mitigating the natural flaws of the defense – then the works were compromised.

While it is easy to sit down with a blank sheet of paper and draw out a defensive arrangement which avoids those extremes, maps of the actual ground to be defended are not blank sheets.  Variations in elevation, watercourses, foliage, buildings, and other features factored into the plan.  You see, this was not a simple exercise in geometry.  That’s why Mahan called the subject he taught “Military Science,” don’t you know!

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 4-5.)

CSS Pee Dee update: Two Brooke Rifles and a Dahlgren pulled out of the PeeDee River

News broke yesterday of the successful recovery of three Civil War cannon from the PeeDee River, all from the gunboat CSS Pee Dee.  In case you missed it, here’s a few of the reports:

South Strand News

The State (Columbia SC)

South Carolina Now

The Florence County museum offered a post and album of photos on their Facebook page.  The guns are 7-inch and 6.4-inch double banded Brooke rifles along with a IX-inch Dahlgren.  All these weapons are of historical significance and rare in their own right.  But the Dahlgren perhaps a little more so.  And one of the photos shows markings from the Dahlgren:


Registry number 513.  That should, if I’m reading my references correct, be from a lot produced by Fort Pitt Foundry.  But what makes this weapon of real interest is that some sources connect it to the USS Southfield, sunk at Plymouth, North Carolina on April 19, 1864.  The Confederates recovered the Southfield’s armament (which also included a 6.4-inch Parrott rifle which I believe was later used in the Cape Fear defenses).  So you might say this Dahlgren has a bit of a story to tell from both sides of the lines.

The articles tell us these guns already have a home:

After conservation, artifacts will be exhibited at the Florence County Veterans Administration building at the Florence National Cemetery.

A win-win, if you ask me.

Summary Statements from Ordnance Reports: The bureaucrats labor is our information gold mine

Working forward from last week’s introduction to Ordnance Reports, as mentioned the individual battery reports were consolidated by the Ordnance Department into summary statements.  While we don’t have a lot of ordnance reports to work from, we do have a fair number of these summary statements.  And these can tell us something about the batteries, their equipment, and general trends in the Federal artillery arm.  It’s information that comes in handy for certain lines of study.  Again, let me thank Brett Schulte for forwarding a copy of the the roll he acquired from the National Archives.

These summaries worked in the way you would imagine any bureaucratic bean-counting record-keeping process.  After receiving the ordnance returns for a given quarter, the Statistical Division of the Ordnance Department extracted the details for entries into a large ledger style book.  Each units’s data spanned across at least twelve pages.  The data from the returns was split into the following classes, considered “Part I” of the summary:

  • Class I: Cannon
  • Class II: Artillery carriages
  • Class III: Artillery implements and equipments
  • Class IV: Artillery projectiles unprepared for service
  • Class V: Artillery projectiles prepared for service
  • Class VI: Small arms
  • Class VII: Accouterments, implements, and equipments for small arms, and horse equipments for cavalry
  • Class VIII: Powder, ammunition for small arms and materials
  • Class IX: Parts or incomplete sets of any articles in Classes I-VIII
  • Class X: Miscellaneous

Following this was Part II, which included tools and materials… and was very lengthy and detailed.  Columns in section for Part II included hammers, punches, and pounds of horseshoe nails.  Yes indeed, the sort of detail that requires a staff of bean-counters three months to compile.  Suffice to say, these large sheets are difficult to demonstrate without straining eyes:


Not to downplay the need for opium for horses (Battery H, 1st US Artillery reported 16 ounces on hand as of December 31, 1862… if you need to know that little tidbit), the stuff most of us are interested in is under Part I, Class I – the cannons.  That class was further subdivided between serviceable and unserviceable cannons, which were even further subdivided by bore type, metal used, and pattern.  The columns included:

  • Bronze smoothbores – 6-pdr field guns, 12-pdr Napoleons, 12-pdr heavy field guns, 12-pdr mountain howitzers, 12-pdr field howitzers, 24-pdr field howitzers, and 32-pdr field howitzers.
  • Iron rifled guns – 3-inch Model 1861 (Ordnance) rifles, 10-pdr Parrotts, and 20-pdr Parrotts.
  • Steel rifled guns – 3-inch types, 6-pdr Wiard, and 12-pdr Wiard.
  • Bronze rifled guns – 6-pdr rifles (3.67-inch), 6-pdr “James” rifles (3.80-inch), and 12-pdr James (4.62-inch).
  • Miscellaneous types – Union repeating guns (Agar Coffee Mill Guns), Bilinghurst-Requa guns, and, written in at times, 4.5-inch siege rifles.

First point to make is that these summaries didn’t track the siege, garrison, or seacoast weapons.  I have not seen a reason for this in writing, but implied is that another mechanism existed to track those type of weapon.  In most cases, the heavy ordnance was issued not to a battery organization but to an installation – be that a fort, garrison, or armory.

Secondly, the field batteries were the place the bean-counters needed the most clarity when accounting for government equipment.  Unlike a fort’s assigned Rodman guns, the Napoleons of a given field battery moved around a lot, sometimes replaced with different weapons, cross leveled or consolidated with other batteries when organizational needs required, and, sometimes, lost in battle.  But that said, I haven’t seen any policy statements from the Ordnance Department as a reference to confirm my speculation.

So we have the header of the first page of the summary with the columns (mentioned above) for the serviceable cannons on hand at time of the report:


And even that section requires reading glasses.  But hopefully you get the gist of this. You see the summary groups the data by regiment.  In this case the 1st Regiment, US Artillery is tabulated by battery, being A through M (there was no J).  Furthermore, you see these were hand written so there are questions about entries.  Things like “is that a four or eleven?” and “is that Murfreesboro or Mumfordville?”   Also, the data needs to be bounced off other sources (such as the official records) for validation.  I’ve run into several issues, such as the annotation of “steel” 3-inch rifles where I know none were in use.

My challenge now is to display this information in a useful format for the web… on a blog post….  A form that 150 years ago would have been Jules Vern crazy talk to the bean-counters in the Statistical Department.

As a start, what I plan to do is post a snip for each regimental organization.  With that I’ll provide what my read is for each.  Then use the comments where questions may be answered and corrections noted.   If successful, then we have a start for a database depicting what batteries had what guns at certain times during the war.